Helical intake ports having a spiral design are frequently used as intake ports in direct injection diesel engines. These helical intake ports set up a strong air swirl inside a cylinder, and promote the mixing of fuel and intake air.
Such a helical intake port is disclosed for example in Japanese Patent Tokkai Sho 59-12124 published by the Japanese Patent Office in 1984. FIG. 8 and FIG. 9 are respectively a vertical cross-sectional view and a plan view of this helical intake port. The helical intake port 10 in these figures comprises a straight passage 16, a swirl passage 13 having a spiral shape and connected to the straight passage 16, and a cylindrical throat 14 formed below the swirl passage 13. Intake air flowing in from the straight passage 16 is given a slewing motion by the swirl passage 13, and flows via the throat 14 from an intake valve 12 into a cylinder 17 while setting up a swirl flow. This prior art, by suitably designing the plan shape of the swirl passage of the intake port, enhances the swirl without reducing the flow coefficient of the intake port.
However, as the characteristics of air flow are determined not only by the plan shape of the swirl passage but also by the cross-sectional shape of same, it is difficult to set a swirl ratio to a desired value and to enhance the swirl and improve the flow coefficient of the intake port at the same time merely by specifying the plan shape of the swirl passage.